Textbox redraw optimization

[ImpulseAdventure]

Optimize the rendering of the scrolling Textbox on slow displays/processors (such as Arduino).

• Goal: minimize flicker when text lines are scrolled up.
• Ideal method: use a graphics RAM copy (within ILI9341) to shift old lines up on the display without having to redraw all characters.
• Alternate optimization: batching of consecutive character options into string operations. One complexity would be in handling color change requests part-way along a row.

[ImpulseAdventure]

Note that I have implemented the "alternate" option (which will be released as part of the updates to #42 ). This mode will be available with the config option #define GSLC_FEATURE_XTEXTBOX_EMBED 0.